This invention relates generally to parallel connection of power transistors, and more particularly relates to the parallel connection of Darlington devices.
For circuits which must handle extremely high power, it is natural to look toward parallel operation of power transistors. This would seem to be a completely straightforward solution to the problem with the power handling capacity increased by parallel connected power transistors. A difficulty with the solution, however, is the fact that transistors of the same type do not always have identical characteristics. Therefore, unless proper precautions are taken, transistors in parallel will not accept equal portions of the load, resulting in damage or even destruction of the circuit. Unsatisfactory solutions to this problem are the use of balancing resistors or the attempted use of matched transistors.
There are a number of methods available for the paralleling of power transistors. One method is to connect the parallel transistors with common collectors and common emitter connections but with separate base drive circuits. This is generally not used, however, because of poor load sharing. Some other acceptable methods are by using separate base drive circuits and balance resistors in the collector or emitter circuits. Another method is the use of the balance resistor in the emitter with a common base connection. The latter three types of circuits are most often used, but the extra resistor power dissipation is a major disadvantage which must be tolerated. However, in the case of a 200-amp Darlington device, this extra resistor dissipation cannot be tolerated because of the high powers involved.
Further, since the successful production of high amperage, compound or Darlington devices, (sometimes referred to as a Darlington transistor or pair) the need for higher current controllers of similar efficiency become evident.
Generally it has not been thought possible to connect Darlington power transistors in parallel, because of differences in the storage time which could result in destruction of one or the pair of power devices. That is, if the storage time of one Darlington device is greater than that of the other Darlington device connected in parallel, then at turn-off the former would carry a full load which could destroy the power device.
Preliminary investigation showed that a degree of static current sharing existed with parallel Darlington devices by virtue of the collector-emitted voltage of each device being proportional to collector current and inversely proportional to heating effect. It was determined that inherent static current sharing capabilities of the Darlington devices was not a serious problem and that the principal problem was the dynamic current sharing with the inductive motor loads.
The static current sharing phenomenon takes place throughout turn-on and full current operation, while the dynamic current sharing problem occurs at turn-off, where in the worst case one Darlington device could momentarily have to handle the entire load. One method for solution to the dynamic current sharing at turn-on would be to place equalizing inductors with individual flyback diodes in the collector circuit of each parallel transistor. However, such inductors are necessarily bulky and add to the cost of the unit, as well as lowering electric vehicle control efficiency through added copper losses.